This invention relates to a heat exchanger tube which can be employed in an absorber, regenerator or evaporator of an absorption refrigerator for producing cold water or of an absorption heat pump for air-conditioner. This invention also relates to a method of manufacturing such a heat exchanger.
The absorber of an absorption refrigerator or of an absorption heat pump for air-conditioner is generally composed of many a number of heat exchanger tubes which are horizontally arranged in rows and in multistage. This group of heat exchanger tubes are adapted to be sprayed from the top thereof with an absorption liquid such as an aqueous solution of lithium bromide.
During the time this sprayed absorption liquid flows down along the outer surfaces of the heat exchanger tubes, the vapor of refrigerant generated from an evaporator is absorbed by the absorption liquid and at the same time the heat generated in the absorption reaction is transferred through heat exchange to a cooling water flowing in the heat exchanger tubes. Therefore, it is imperative for improving the performance of the absorber to promote the phenomenon of mass transfer in this process of absorbing the vapor of refrigerant.
At the moment when an aqueous solution of lithium bromide absorbs the vapor of refrigerant, a mass transfer as shown in FIG. 1 takes place at an interface between the absorption liquid A and the refrigerant vapor B. Namely, at the surface layer Aa of the absorption liquid A, i.e. at the interface layer between the absorption liquid A and the refrigerant vapor B, the concentration of the absorption liquid A becomes thinner as compared with that of inner layer Ab of the absorption liquid A which is close to the surface of the heat exchanger tube C. Accordingly, if the absorption of the refrigerant vapor B is to be promoted, the turbulence of the absorption liquid A on the heat exchanger tube C is required.
Because of this, in case of the absorption refrigerator or the absorption heat pump for air-conditioner which is actually used now by making use of an aqueous solution of lithium bromide, a surfactant such as n-octyl alcohol or 2-ethyl-1-hexanol is added at a concentration of several tens to several hundreds ppm to an aqueous solution of lithium bromide so as to cause a turbulence action (which is called Marangoni convection) in the absorption liquid in the aforementioned process of refrigerant vapor absorption. Namely, a method of improving the refrigerant vapor absorption capacity of the absorption liquid by making the most of this Marangoni convection is generally adopted now.
Accordingly, it is now desired, in view of improving the performance of the heat exchanger tube of an absorber, to effectively promote the turbulence by way of Marangoni convection of an absorption liquid layer on the outer surface of the heat exchanger tube, which would be generated as mentioned above as the absorption liquid absorbs the refrigerant vapor.
A heat exchanger tube which is designed to promote the turbulent action in the absorption liquid has been proposed by Japanese Utility Model Unexamined Publication S/57-100161. The heat exchanger tube disclosed in this Japanese Utility Model Unexamined Publication S/57-100161 is worked such that fine spiral grooves are formed on the outer surface thereof. The purpose of providing the spiral grooves is to allow an absorption liquid to flow along the spiral grooves so as to spread the flow of the absorption liquid on the surface of the heat exchanger tube and at the same time to promote the turbulence in the absorption liquid layer by the irregular surface which has been effected by these spiral grooves.
Another example of heat exchanger tube which is also designed to promote the turbulent action in the absorption liquid has been proposed by Japanese Utility Model Unexamined Publication S/64-35368. The heat exchanger tube disclosed in this Japanese Utility Model Unexamined Publication S/64-35368 is provided on the outer surface thereof with fine spiral grooves, i.e. a first kind of spiral grooves and a second kind of spiral grooves twisted in a direction opposite to that of the first kind of spiral grooves, thus forming protrusions which are formed by the intercrossing of these two sets of spiral grooves. The purpose of providing two sets of spiral grooves is to allow an absorption liquid to impinge against the protrusions formed by the intercrossing of these grooves so as to promote the turbulence in the absorption liquid.
In the case of the heat exchanger tube described in this Japanese Utility Model Unexamined Publication S/57-100161, it is certainly possible as shown in FIG. 2A to spread the flow of the absorption liquid layer on the surface of the heat exchanger tube C1 due to the presence of the spiral grooves V1. However, since the spiral grooves V1 is linear, the turbulence of the absorption liquid to be obtained would be insufficient.
On the other hand, in the case of the heat exchanger tube described in the Japanese Utility Model Unexamined Publication S/64-35368, the absorption liquid layer A2 is impinged upon a protrusion E1 thereby to generate a turbulence. However, since these two sets of spiral grooves V2 and V3 are twisted in an opposite direction from each other in relative to longitudinal direction of the tube and hence intercrossed with each other, the turbulence of absorption liquid layer A2 generated by the protrusion E1 is caused to collide with the turbulence of absorption liquid layer A3 generated by the protrusion E2 disposed next to the protrusion E1. As a result, it is impossible to maintain the turbulence of the absorption liquid layers A2 and A3 along the longitudinal direction of the tube, thereby making it difficult to effectively promote the turbulence of the absorption liquid. Therefore, it is difficult to maintain the turbulence of the absorption liquid layers A2 and A3 on the surface of the heat exchanger tube C2 for a long period of time.
On the other hand, in the case of an absorption refrigerator or an absorption type hot and cold water generator, a cold water is produced by taking the latent heat of vaporization of a refrigerant from a water to be cooled when the refrigerant is vaporized from an evaporator. The vaporized refrigerant from the evaporator is then absorbed by an absorption liquid in an absorber so as to be turned back to a liquid state while releasing the latent heat of vaporization and the heat of dilution.
Since the absorption of refrigerant becomes more difficult with a rise in temperature of an absorption liquid, the absorption liquid is required to be cooled by the surface of a heat exchanger tube, thereby inhibiting the absorption liquid from heated excessively by the latent heat of vaporization and the heat of dilution.
Generally, the ordinary absorber is constructed such that many a number of heat exchanger tubes are arranged horizontally or vertically and an absorption liquid is allowed to flow down along the surfaces of these heat exchanger tubes in which a cooling water is circulated. The heat exchanger tube is generally constructed of a plain tube unless there is any special requirement to employ a high performance heat exchanger tube for the purpose of enhancing the performance of the tube.
For improving the performance of heat exchanger tube in an absorber, the following countermeasures are required to be taken.
(1) To increase the heat exchange area;
(2) To minimize the non-uniformity in concentration between the upper layer and the lower layer of the absorption liquid layer, which is caused from the absorption of vapor by the surface of the running absorption liquid and the resultant thinning in concentration of the absorption liquid; and
(3) To promote the interfacial turbulence of the absorption liquid flowing down along the surface of the heat exchanger tube.
One example of such a high performance heat exchanger tube is proposed in Japanese Utility Model Unexamined Publication S/57-100161, wherein many a number of fine spiral grooves are formed on the outer surface of the heat exchanger tube. Another example of such a high performance heat exchanger tube is proposed in Japanese Utility Model Application S/58-51671, wherein many a number of fine spiral grooves of the same depth intercrossing with each other are formed on the outer surface of the heat exchanger tube. Still another example of such a high performance heat exchanger tube is proposed in Japanese Utility Model Unexamined Publication H/1-73663, wherein many a number of fine spiral grooves which are intercrossed with each other are formed on the outer surfaces of only the end portions of the heat exchanger tube.
It is admitted that the performance of a heat exchanger tube can be improved by forming many a number of fine spiral grooves on the outer surface of the heat exchanger tube; by forming many a number of fine spiral grooves on the outer surface of the heat exchanger tube in such a manner that these spiral grooves intercross with each other; or by forming many a number of intercrossed fine spiral grooves on the outer surfaces of only the end portions of the heat exchanger tube.
However, the method of forming many a number of fine spiral grooves in the same depth and in the same direction on the outer surface of the heat exchanger tube, as well as the method of forming many a number of intercrossed fine spiral grooves on the outer surfaces of only the end portions of the heat exchanger tube, as disclosed in Japanese Utility Model Unexamined Publications S/57-100161 and H/1-73663, are accompanied with a problem that the flow of the absorption liquid on the surface of the heat exchanger tube becomes unidirectional so that it is difficult to achieve a sufficient interfacial turbulence of the absorption liquid, which is one of the aforementioned requirements to improve the performance of heat exchanger tube.
On the other hand, the method of forming many a number of fine spiral grooves of the same depth on the outer surface of the heat exchanger tube in such a manner that these grooves intercross with each other as suggested in Japanese Utility Model Application S/58-51671 is also accompanied with a problem that since the flow of the absorption liquid simply runs down along the bottom of the spiral groove, the spread of the flow of the absorption liquid in the longitudinal direction of the heat exchanger tube is not so promoted, and it is difficult to minimize the non-uniformity in concentration between the upper layer and the lower layer of the absorption liquid layer.
Because of these reasons, the improvement in heat exchange property of the absorber is still insufficient even if the aforementioned heat exchanger tubes are substituted for the plain tube.